Connection Help needed!

I have an airport extreme that a specific device can connect to, but loses the ability to browse after a minute or so. I added an airport express to a part of my house that needed better coverage, and the same device can connect to that and work at length with no problems, but as soon as I go back to the main part of the house where it hands over to the main (extreme) router, the problem comes back. How is that even possible considering the express is just repeating the extreme's connection.

I have 15-20 other devices at any given time connected, and none of them have the issue at all.